The present invention is directed to the fabrication of bipolar transistors, and in particular is concerned with a fabrication technology which is known as the "bipolar single poly process."
This technology is basically characterized by the use of a single polysilicon layer which forms the device contact to each of the collector, base and emitter of the transistor, as well as to provide local connection among transistors. The technology is based upon 2 .mu.m (nominal) design tolerance rules and offers gate delays in the range of 170-200 psec. Among the reasons for its popularity are the fact that it provides self-aligned device contacts, self-aligned local interconnects and polysilicon emitters, which afford enhanced yields and high gain transistor action.
A limiting factor in the speed that can be obtained with devices manufactured according to this technology is the size of the emitter. It is desirable to make the emitter as small as possible, to thereby reduce parasitic capacitances and increase the speed of operation of the transistor. In order to reduce the size of the emitter below that available with conventional optical equipment that affords design dimensions in the range of 1-2 .mu.m, it becomes necessary to use exotic photolithography equipment, such as x-ray or electron beam devices.